1. Field of the Invention
The present invention relates to a process for producing an oxide crystal thin film.
2. Description of the Related Art
Two types of processes for producing an oxide crystal thin film are known, i.e., a process in which a mixture of component powders is fired at a high temperature, and a process in which a crystallization is simultaneously effected during deposition on a heated substrate by vapor deposition or sputtering.
In a multiple component oxide, however, phases having close free energies of formation grow simultaneously to form a mixture of phases containing undesired phases, and thus a required property cannot be obtained. Moreover, particularly in a thin film, a three dimensional nucleation "Q" occurs on a substrate "P", as shown in FIG. 4, which causes an uneven film surface and makes it impossible to provide a good thin film having a flat surface.
The mixed phases and the three dimensional nucleation are a serious defect in materials such as oxide superconductor thin films having properties which are adversely affected by these phenomena.
The oxide superconductor has a very high critical temperature, Tc, in comparison with that of metal alloy superconductors, and therefore, is thought to be suitable for various applications, including conductors in memories and logic circuits, etc., of computers, superconductor magnets, superconducting cables, superconducting electric power storage, and other general electrical wiring, and for other applications thereof including magnetically levitated trains, accelerators, and medical devices, etc.
The oxide superconductor is composed of numerous component elements, and the superconductive properties including the critical temperature are very sensitive to the chemical composition of the oxide, and thus an inclusion of phases other than a single desired phase of a predetermined chemical composition must be prevented. Moreover, particularly in a thin film, a three-dimensional nucleation, which causes an uneven film surface, must be prevented at all costs.
Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10, for example, is a high temperature superconductor having a highest level of Tc (110K) among known superconducting substances, but when producing Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 by a process including a firing of a powder mixture, a problem arises in that a solid state reaction induced by the heating for the firing causes a ready formation of a mixture of three phases having close free energies of formation, i.e., a Bi.sub.2 Sr.sub.2 Cu.sub.1 O.sub.6 phase having a Tc of 7K, a Bi phase having a Tc of 80K, and =Bi a Tc of 110K, and therefore, the characteristic excellent properties expected from a single Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 phase cannot be obtained. Moreover, another problem arises in that a thin film having a flat surface cannot be obtained, because a three-dimensional nucleation readily occurs.
The same problems arise, for example, in a process for producing an oxide superconductor coating as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 1-167912, in which a RBa.sub.2 Cu.sub.3 O.sub.7-x type oxide superconductor is vapor-deposited on an MgO crystal substrate heated at a temperature of 500.degree. C. or lower, to form an amorphous film which is then heated to a temperature of from 800.degree. to 950.degree. C. to be crystallized, followed by a heating at a temperature of from 400.degree. to 550.degree. C. to transform the film into a superconductor.
Another process was proposed by J. N. Eckstein et al. in the Journal of Vacuum Science Technology, B7(2), March/April 1989, pages 319-323, in which oxide superconductor thin films, such as Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10, are formed by a continuous deposition on a substrate heated and held at a temperature high enough to induce a simultaneous crystallization of a deposit on the substrate. In this process, the deposition is carried out in such a manner that atomic layers having predetermined chemical compositions are deposited layer-by-layer on the substrate, and that each deposited atomic layer is immediately crystallized upon contact with the heated substrate, i.e., simultaneously with the deposition thereof. This process, however, does not consider the prevention of an occurrence of an uneven film surface due to a three-dimensional growth, which often occurs at a substrate temperature high enough to induce the simultaneous crystallization of the atomic layer when deposited on the substrate.